1. Field of the Invention
The present invention relates to a lithium secondary battery, and, more particularly, to an improved structure of connection between an electrode tab and a protection circuit board electrically connected thereto, and a pouch-type lithium secondary battery having the improved connection structure and a fabrication method thereof.
2. Description of the Related Art
With the development of portable electronic devices such as cellular phones, notebook-type computers, camcorders, and the like, increased research into secondary batteries generally capable of charging and discharging is being conducted. Lithium batteries are classified into nickel-cadmium (Ni—Cd) batteries, nickel-hydride (Ni-MH) batteries, and lithium secondary batteries. Specifically, the lithium secondary batteries are being rapidly developed in view of their high operating voltage of 3.6 V or higher, which is approximately 3 times that of the nickel-cadmium (Ni—Cd) batteries or nickel-hydride (Ni-MH) batteries, and due to their excellent energy density per unit weight.
The lithium secondary batteries may be classified into liquid electrolyte batteries and solid electrolyte batteries, according to the electrolyte used. In general, batteries using a liquid electrolyte are referred to as lithium-ion batteries, and batteries using a polymeric electrolyte are referred to as lithium polymer batteries.
The lithium secondary batteries may be manufactured in various shapes, and are typically formed in cylindrical and rectangular shapes. Lithium polymer batteries, which have attracted much attention recently, are manufactured in a flexible pouch shape so that they are relatively free in view of shape design. Also, lithium polymer secondary batteries, which have excellent safety characteristics and are light in weight, are advantageous in attaining miniaturized and lightweight portable electronic apparatuses.
FIG. 1 shows a conventional pouch-type lithium secondary battery 10, and FIG. 2 is a partially extracted cross-sectional view of the lithium secondary battery shown in FIG. 1. Referring to the drawings, the lithium secondary battery 10 includes a battery unit 11, a case 12, and a space 12a, provided in the case 12, in which the battery unit 11 is accommodated.
The battery unit 11 includes a positive electrode plate 13, a negative electrode plate 14, and a separator 15 interposed therebetween. The positive electrode plate 13, the separator 15, and the negative electrode plate 14 are stacked in sequence. In such a state, the stacked structure is laminated and wound in a jelly-roll type structure.
The positive electrode plate 13 and the negative electrode plate 14 are electrically connected to a positive electrode tab 16 and a negative electrode tab 17, respectively. The positive and negative electrode tabs 16 and 17 extend a predetermined length outward from a sealing surface 12b of the case 12. A portion where the positive electrode tab 16 and the negative electrode tab 17 are electrically connected to the sealing surface 12b is wrapped by an insulating tape 18, thereby increasing sealing efficiency.
Unlike the cylindrical or rectangular lithium secondary battery, which use a thick metal plate as a packaging material, the case 12 is preferably formed in a pouch type laminate constructed with a medium layer made of a thin metal foil, and inner and outer coatings fixed to both surfaces of the medium layer, respectively, thereby providing bending flexibility.
The positive and negative electrode tabs 16 and 17, extending outside the sealing surface 12b, are electrically connected to a protection circuit board 100. A safety element, such as a positive temperature coefficient (PTC) element, is provided in the protection circuit board 100. The positive and negative electrode tabs 16 and 17 are respectively connected to positive and negative electrode terminals 101 and 102 provided in the protection circuit board 100. Thereafter, the protection circuit board 100 is mounted on the sealing surface 12b of the case 12.
However, the conventional lithium secondary battery 10 has several problems. Unlike the cylindrical or rectangular lithium secondary battery, the pouch-type lithium secondary battery 10 requires a predetermined width of the sealing surface 12b of the case 12 to mount the protection circuit board 100 thereon. In order to mount the protection circuit board 100 on the sealing surface 12b, the positive and negative electrode tabs 16 and 17, extending outward from the sealing surface 12b, are bent at least once.
In other words, as shown in FIG. 2, the positive and negative electrode tabs 16 and 17 are bent in an S-shaped form such that edges of the positive and negative electrode tabs 16 and 17 are bent once at portions where they extend outward from the sealing surface 12b, and are then bent again in the opposite direction. The protection circuit board 100 is seated on top of the thus-bent positive and negative electrode tabs 16 and 17.
In order to mount the protection circuit board 100 in such a manner, it is necessary for the sealing surface 12b to have a predetermined width W1 corresponding to a width of the protection circuit board 100.
In recent years, the battery unit 11 of the lithium secondary battery 10 has become larger in order to obtain a large capacity lithium secondary battery. Thus, the space 12a in which the battery unit 11 is accommodated has tended to increase, while minimizing the area of the sealing surface 12b. This has introduced a critical problem of developing efficient methods of attaching the protection circuit board 100 to a limited area of the sealing surface 12b. 
Because the positive and negative electrode tabs 16 and 17 are bent in an S-shaped form, they may be easily cut at bent portions during a bending process. Also, at portions where the positive and negative electrode tabs 16 and 17 extend outward from the sealing surface 12b of the case 12, the positive and negative electrode tabs 16 and 17 may be electrically connected to a medium layer exposed along the edge of the case 12 by a cutting process, resulting in a short-circuit.